"Most cichlids dig, it gives them something to do." Yeah, like most of us go to work?
First and foremost, the multies are digging to find security, then they'll seek to establish a home to raise fry in. From what you say they have not taken up residency in the barnacles so they'll simply dig a pit to lie low in. Are the leleupi dominating the tank at the moment, are the barnacles in the leleupi's territory? Can you rearrange to keep the multies and leleupi apart, maybe rockwork at just one end and open substrate elsewhere?
Once they have a shell or shells (they like a choice) of their own, they'll dig around it until it reaches the bottom on the tank. They'll pile up the sand to construct ramparts around the shell/ or shells to mark the borders of their territory then the dominant male will defend it from all comers as best he can.
Multies will form a pair or a dominant male may have a harem of females.